1. Field of the Invention
The present invention relates to small extrusion dies which are suitable for the production of small objects for use in electrical, automotive and related manufacturing industries, and a method for their use. Such objects are multi-cavity flat tubes made of aluminum and aluminum alloys which are used in heat exchangers, for example, evaporators for automotive air-conditioners, condensers, radiators and other related products. The features of the invented dies are that they are easier to manufacture compared with the conventional extrusion dies and that they provide a long service life, by maintaining the dimensional accuracy even after a prolonged use.
2. Background Art
Extrusion process is explained first with reference to FIG. 1. In general, a process of forming an object by extrusion includes the following steps: placing a billet 51 in a receptacle means, generally referred to as a container 50; pressing the billet 51 with a stem 53 toward an exit opening 54 or a depressed section; flowing the material constituting the billet 51 through a space defined by the opening 54 (die cavity), and in some cases also by a mandrel which is inserted into the opening, in which the space formed in between the opening and the mandrel is shaped in a shape of the profile of the object desired. Through such a process, an object having the desired cross sectional shape is obtained.
One important feature of the extrusion process is that a product of a very complex cross sectional shape can be obtained through one processing step of exerting a compressive pressure around the billet placed in the container, and squeezing the material out of the shaped die cavity.
For this reason, extrusion process is applied also to forming of aluminum alloys to produce multi-cavity flat tubes for use in heat exchangers such as evaporators for automotive air-conditioners, condensors and radiators.
In the following, the features of the present invention will be explained with particular reference to the production of multi-cavity flat tubes by the conventional extrusion process and by the method of the present invention. However, it will be understood that the present invention is by no means limited to this particular application.
FIG. 2 illustrates the shape of a multi-cavity flat tube produced by the above presented conventional extrusion process. Such process is disclosed in Japanese Patent Application First Publication JPA S64(1989)-31571 and Japanese Utility Model Application, Second Publication JUA H3(1991)295.
Extrusion dies suitable for the production of such shapes are known to be integral bridge dies or insert dies.
FIG. 3 illustrates an example of the integral bridge type dies. Such a die 60 is composed of a cylindrical body which includes the bridge part for supporting a female die and male die which are formed integrally with the rest of the die body. There is a die cavity 61 which run through the die 60 parallel to the die axis from one surface to the opposite surface of the die 60. The cavity 61 forms an integral part of the shape forming opening which is constituted by the male and female dies. Therefore, such a die is composed of a die body which includes both male and female dies within one body, and may be made up of a number of sections. (In the case of the die shown in FIG. 3, the die body has four sections.) For such a die, if one section of the die is damaged, the entire die body becomes defective, because it is not possible to replace one section of a die body, and in some cases, the die 60 itself may have to be replaced.
To overcome such problems associated with the bridge dies, insert type dies were developed. With reference to FIG. 4, an insert die is composed of a die holder 80, and at least one die body 70 which can be inserted into or taken out of the die holder 80 freely. The die holder 80 usually has a plurality of openings 81 for receiving a die body 70 in each opening 81. The die body is comprised of two engaging cylindrical sections. The first section (female type) has a certain cavity shape, and the second section (male type) has a protrusion of another shape which is inserted into the first section. Therefore, if the cavity of one of the die bodies 70 becomes defective, it is necessary to replace only the section damaged or only the die body 70 concerned.
The construction of the die body 70 of the insert dies will be explained in more detail, with reference to FIGS. 5 to 7. In all the descriptions which follow, the surfaces and directions are referenced with respect to the direction of travel of the material being extruded. In the case of FIG. 5, the billet is placed against the second section 70b (referred to as the male section 70b), and is extruded toward the first section 70a (referred to as the female section 70a). The entry-side is defined as the side from which the material enters the die, and the exit-side is defined as the side from which the material leaves the die.
Generally the die body 70 is a roughly cylindrical body as shown in FIG. 5 and consists of two parallel sections 70a and 70b whose flat surfaces are disposed transverse to the axis of the die body 70. The first section 70a has two concentric parts: an outer depressed part 71 of a large circular shape (female mating part whose internal wall surface 71a fits with the wall surface of the male section which will be described later); and an inner depressed part 72 having a four leaf shape, which is made by machining out the central portion of the outer depressed part 71. An elongated opening 73 is formed along a diametrical axis of the section 70a. With reference to FIG. 6, the female section opening 73 consists of an extrusion cavity 73a at the exit-side of the four leaf part 72, and an exit region 73b which has a larger opening than the die cavity and which communicates with the entry-side surface of the section 70a. In this particular example, the cross sectional shape of the die cavity 73a transverse to the die axis is shown, in FIG. 5, to be a wide slit with the corners rounded. It is also shown in the same figure that there is a pair of locating holes 75 disposed diametrically opposite to each other, and a pair of threaded holes 74 which are disposed similarly.
The second section 70b is provided with a male mating part 76, on the exit-side surface, to fit with the female mating part 71a described above, along all its periphery. There is an integrally formed comb-shaped part 77 (FIG. 5), which extends along parallel to the female section opening 73, and comprised of a plurality of protrusions. The comb-shaped part 77 functions as a mandrel when inserted into the die cavity 73a of the section 70a. The male section opening 78 is formed along the extrusion direction following the contours of the comb-shaped part 77. The male section opening 78 communicates with both the entry-side surface and the exit-side surface of the second die section 70b. When the die sections 70a and 70b are joined together, the male section opening 78 forms a container and acts as the billet chamber in conjunction with the four leaf shaped depressed part 72 of the second section 70a. To prevent misalignment of the two sections, 70a and 70b of the die body 70, two locating pins 80 are made to align with the two locating holes 75, and the threaded holes 79 of the second section 70b are aligned, respectively with the threaded holes 74 of the first section 70a.
Manufacturing of the male dies is performed using the methods which are routine to those skill in the field of extrusion. The processing includes the following steps:
1. Machining such as lathe cutting and drilling which requires the use of cutting bits; PA1 2. Heat treatments, including hardening; PA1 3. Polishing; and PA1 4. Electric discharge machining (EDM): after the hardening heat treatment process above, the dies cannot be machined by the cutting bits, so the dies are fabricated by means of electric arc discharge from electrodes such as Cu electrode while washing off the debris formed by the discharge with oil. PA1 5. Wire discharge cutting which is a type of EDM.
There are serious problems associated with such processing steps mentioned above, in particular, the lathe and milling operations require a large number of processing steps and are time consuming. Approximately twenty steps, over a period of about ten hours, are required from the start to the completion of making a male die. Female dies also require about the same number of steps over a period of about six hours. Practical steps necessary would be evident to those skilled in the art from the complex shape of the die sections illustrated in FIG. 5.
There are additional problems in the case of the insert dies as described below.
(i). The size of the entry port for aluminum extrusion is set by experience, on the basis of the die strength. However, the required cross sectional area is relatively small for most aluminum extrusions, and the required extrusion pressure is high in relation to the strength of the die material. High stresses are imposed on the die, and consequently, the die suffers slight permanent distortions.
Such distortions affect the precise fitting of the two sections (male and female sections) of the die, resulting in the loss of dimensional accuracy of the product. The accuracy of alignment due to pins and screws is also affected. Even if one section is replaced with a new section, the combination of new and old dies cannot reproduce the original dimensional accuracy. When the distortion is allowed to continue, the die must eventually be discarded.
(ii) Because the dies are made of two separate sections, alignment devices such as pins and screws are required. It is necessary to fabricate such parts, but it is difficult to attain the precision required for the pin holes and threaded holes by lathe machining. Wear is introduced during the operation, because every time a die is disassembled or assembled the pins are removed or driven into the dies, thereby accelerating the loss of service life of the die.
(iii) Heat treatment processes are required which introduces thermal distortions in the dies, making it difficult to maintain the required precision, and because of the complexity of the die shape, it is difficult to completely correct such distortions.
(iv) Many machine shops making extrusion dies lack the ability to accurately measure the internal diameter of the female die section, thus making it difficult to manufacture a high precision joint part by lathe machining.
(v) To improve wear resistance, it is desirable to coat the surfaces of the die with known abrasion resistant coating, but it is difficult to coat the die structure, including the pin holes, uniformly with the applicable coating techniques. If the coating thickness in the interior surfaces of pin holes becomes non-uniform, the alignment accuracy of the male and female dies becomes poor.
(vi) The suitable die materials include such hard materials as heat treatable tool steels and highspeed steels. However, because of the large size of the most insert dies, it is not preferable to make insert dies with such hard materials which are susceptible to cracking. The forces responsible for causing such cracking in insert dies arise from the impact of initial loading as well as from the extrusion process.
Therefore, there has long been an outstanding need for the development of durable extrusion dies which provide long service life without causing fracture, wear and distortions due to extrusion processes, which accept coating processing uniformly and easily thereon and which provide a long service life by maintaining the initial machining precision of the die components.